CHAPTER 1

Seabed Characteristics

Most discussions on anchoring begin with the anchor itself, but we believe itmakes more sense to look first at the seabed in which you want your anchor tohold, and then look at the forces that can make your anchor drag or dislodge.This is how we have approached the discussion in this book, and we hope thefirst two chapters provide a solid context for the discussion in Chapter3 on anchor selection.

The seafloor is one of the most overlooked aspects of anchoring. No responsibleskipper disregards visible problems, such as a loose cleat or a worn link in ananchor chain, but what remains unseen is often ignored. What you can't see,however, can of course hurt you, so it's essential to pay attention to thenature of the bottom in which you hope to set your anchor.

Unfortunately, not all anchorages offer large areas with excellent holdingground in dense clay or fine sand. Seafloor characteristics can vary greatlywithin a few feet. Two boats side by side in an anchorage may easily have theiranchors in different sediments, and even when the sediment itself ishomogeneous, you may encounter a slippery forest of dense weed just a few feetfrom a very good-holding sand patch. Asking your neighbor how the holding is ina particular anchorage may contribute to your decision making, but you can neverreally know the quality of any given anchor ground with absolute certaintyunless you retrieve a sample or dive down and look for yourself!

Sometimes crystal-clear water will offer a view of what you're sinking into,even in deep water. Most seasoned mariners learn to read the chiaroscuro mosaicbelow, aiming for the lighter patches of sand between the dark boulders,seaweed, or other impenetrable spots.

But your eyes will only help so much. A good view of hard, compact sand may lookthe same as the loosely packed spot a few feet away. What's more, even with anexcellent view of your anchoring field, that's only the upper layer. Once ananchor has pierced the bottom surface, it should dig itself into the subsurfacelayer. A sandy surface may only be a few inches deep, hiding an impenetrablerock plate below.

If you can see your anchor safely tucked in the seabed, you may feel safe inyour bunk bed, too. But be aware that even a completely buried anchor is noguarantee; soft mud, seashells, and pebbles offer only precarious holding.

Nautical charts, cruising guides, and pilots often give helpful information onseabed characteristics in a particular area, so you should always check yourchart when preparing to anchor to see what it says about the type of bottom; seeTable 1-1 for a list of abbreviations. This information, however, may beinaccurate or may not represent your exact anchoring spot. So if you don't wantto get out your mask and snorkel, do what the ancient mariners did: heave asounding line.

A sounding line or lead line is a length of rope with a leadweight at the lowered end. Used to measure depth, this handy device also allowsyou to check seafloor characteristics. Put tallow, wax, or grease on the bottomof the lead weight to pick up traces of mud, sand, or shingle from the seabed.If you don't have a lead line, put some grease on your anchor, lower that, thenbring it back up to see what's sticking to the flukes. This will work, but alead line is much easier!

TYPES OF SEAFLOORS

Thanks to the meticulous work of cartographers, surveyors, and marinegeologists, we have access to fairly accurate data pertaining to the seafloorsediments near the coastlines of the world. Geologists have classified theseabed into four major categories by particle size: muds, sands, gravels, androcks. Keep in mind that these classifications are complicated by the fact thatseafloors rarely have a homogeneous surface. Sand can be muddy, covered inalgae, or contain a greater or lesser proportion of shell or coral fragments.The various types of seafloors differ in their penetrability and their capacityfor holding anchors.

Mud

Geologists define mud as consisting of particles smaller than 62.5 microns indiameter. We can't see a particle this small with the naked eye, since it takes1,000 microns to make 1 millimeter and 25.4 millimeters to equal 1 inch. Mud issubdivided into clay particles, which are smaller than 4 microns in diameter,and silt particles, which are 4 to 62.5 microns.

A semiliquid mixture of water and sediment, mud is soupier and lighter than pureclay, and less sticky. The softer and soupier mud is, the weaker its holdingcapacity will be.

Sand

Sand is comprised of rock that has been abraded by wave action into particles,or granules, ranging in size from 0.063 mm (63 microns) to 2 mm. Sand can befurther subclassified as fine sand (0.06 mm to 0.2 mm), medium sand (0.2 mm to0.6 mm), and coarse sand (0.6 mm to 2 mm). A mixture of fine and medium sand isconsidered dense sand for purposes of holding (see Table 1-2). Note thateven the coarsest grain of sand is no more than a tenth of an inch in diameter.

Some sands are made up of the skeletal material of marine organisms. Coral sandis created not only by wave action but also by bio-erosion. For example, parrotfish bite off pieces of coral, digest the living tissue, and excrete theinorganic component as silt and sand. Coral sand is one of the better materialsin which to sink your anchor. Long live the parrot fish!

Gravel and Rocks

The next coarser sediment class above sand is gravel, with particles rangingfrom 2 mm to 60 mm (i.e., from 1/10 inch to roughly 2.5 inches). We cansubclassify gravel into granules (2 mm to 6 mm), pebbles (6 mm to 20 mm), andstones (20 mm to 60 mm). Since it has little cohesion, gravel is one of theworst materials in which to anchor. It can also prevent an anchor from settingeven when it covers a more desirable sediment such as fine sand. You can onlyhope that if your anchor digs itself in deeply enough, the sheer weight of thegravel will keep it in place.

Above 60 mm (2.5 inches) in diameter, we have rocks, then boulders. (In theUnited Kingdom, cobbles are considered larger than pebbles and smaller thanboulders, in the size range of stones and rocks.) Rock bottoms offer no holdingpower at all, unless you get lucky enough to hook a...